Presynaptic mitochondria in functionally different motor neurons exhibit similar affinities for Ca2+ but exert little influence as Ca2+ buffers at nerve firing rates in situ.
Presynaptic mitochondria in functionally different motor neurons exhibit similar affinities for Ca2+ but exert little influence as Ca2+ buffers at nerve firing rates in situ.
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DOI：
10.1523/jneurosci.4701-09.2010
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发表时间：
2010-02-03
期刊：
影响因子：
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通讯作者：
Macleod GT
中科院分区：
文献类型：
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作者：
Chouhan AK;Zhang J;Zinsmaier KE;Macleod GT
线粒体在神经末梢内积聚并支持突触功能，最显著的是通过产生ATP。在神经刺激过程中它们也能隔离Ca²⁺，但在生理神经放电频率下，这是否会限制突触前Ca²⁺水平尚不清楚。同样，在功能不同的神经末梢之间，线粒体对Ca²⁺的隔离是否存在差异也不明确。我们结合合成的和基因编码的Ca²⁺指示剂（GECIs）来研究果蝇紧张性（MN13 - Ib）和位相性（MNSNb/d - Is）运动神经元突触前末梢的胞质和线粒体Ca²⁺水平，我们确定在虚拟运动过程中它们分别以约42Hz和约8Hz的频率放电。线粒体对Ca²⁺的隔离在两种末梢中均在约250nM时开始，表现出相似的Ca²⁺摄取亲和力（约410nM），并且不需要内质网释放Ca²⁺。尽管如此，Is型末梢中的线粒体Ca²⁺摄取对低频神经刺激更敏感，这是由于其胞质Ca²⁺水平较高。由于Ib型末梢的线粒体密度高于Is型末梢，似乎线粒体对Ca²⁺更大的隔离可能是Ib型末梢胞质Ca²⁺水平较低的原因。然而，在生理相关的刺激频率范围内，对线粒体Ca²⁺摄取进行基因和药理学操作并没有显著改变两种末梢类型中神经刺激引起的胞质Ca²⁺水平升高。我们的研究结果表明，在功能不同的神经末梢中，突触前线粒体对Ca²⁺具有相似的亲和力，但在原位运动神经元放电频率范围内不会限制胞质Ca²⁺水平。
Mitochondria accumulate within nerve terminals and support synaptic function, most notably through ATP production. They can also sequester Ca2+ during nerve stimulation, but it is unknown whether this limits presynaptic Ca2+ levels at physiological nerve firing rates. Similarly, it is unclear whether mitochondrial Ca2+ sequestration differs between functionally different nerve terminals. We addressed these questions using a combination of synthetic and genetically-encoded Ca2+ indicators (GECIs) to examine cytosolic and mitochondrial Ca2+ levels in presynaptic terminals of tonic (MN13-Ib) and phasic (MNSNb/d-Is) motor neurons in Drosophila, which, as we determined, fire during fictive locomotion at ∼42 Hz and ∼8 Hz, respectively. Mitochondrial Ca2+ sequestration starts in both terminals at ∼250 nM, exhibits a similar Ca2+-uptake affinity (∼410 nM), and does not require Ca2+ release from the endoplasmic reticulum. Nonetheless, mitochondrial Ca2+ uptake in type-Is terminals is more responsive to low frequency nerve stimulation and this is due to higher cytosolic Ca2+ levels. Since type-Ib terminals have a higher mitochondrial density than Is terminals, it seemed possible that greater mitochondrial Ca2+ sequestration may be responsible for the lower cytosolic Ca2+ levels in Ib terminals. However, genetic and pharmacological manipulations of mitochondrial Ca2+ uptake did not significantly alter nerve-stimulated elevations in cytosolic Ca2+ levels in either terminal type within physiologically relevant rates of stimulation. Our findings indicate that presynaptic mitochondria have a similar affinity for Ca2+ in functionally different nerve terminals, but do not limit cytosolic Ca2+ levels within the range of motor neuron firing rates in situ.